DC for the Hands-On Technician

Course Description

This seminar is geared to the hands-on technician, as well as those with an interest in DC repair. It will be presented in a visual format, with no math or theory. Well supported by slide format. We will have DC motors in various stages of disassembly to be used as work pieces in conjunction with West Fraser Electro/Mechanical’s repair facility and equipment as classroom training tools.

Course Outline

How a DC motor works, in layman’s terms

  • Opposites attract
  • Field flux distortion: Interpoles & Compensating windings
  • Function of Fields: Interpoles / Compensating Windings
  • What is Brush Neutral Position?


  • Visual inspection
  • Commutator condition
  • Brushes: What to check, and why
  • Bearing inspection
  • Shaft currents / axial loading / misalignment
  • Brushholders & Springs
  • Minimum Commutator Diameters

Incoming tests

  • Drop test: AC vs. DC
  • Armature testing: Surge test / Growler / Monolithic / Ductor
  • Testing fields & interpoles: Surge test / Current comparison / Resistance measurement

Brush Neutral

  • Brush seating: Tips to make the job easier
  • Neutral defined with graphics
  • AC method for setting neutral
  • Special cases; Back-turns, C-lead V. Assembly testing
  • Cumulative / Differential
  • Check for correct Interpole/armature relative polarity
  • Brush spacing
  • No-load test run


  • Reading a commutator
  • Wear pattern & what they mean
  • Unusual brush wear: symptoms and solutions

Turn & Undercutting commutators

  • Blade selection
  • Depth of undercut
  • Chamfering tips Limitrac; reducing flashovers

Armature Stripping Tips

  • Heat vs. Cold stripping
  • Eddy currents & armature
  • Effects of excess heat on armature
  • Cost of Labor
  • An effective method for cold-stripping armatures without damage
  • Contamination of air supply: silicon / hydrochloric acid /chlorine
  • Limitrac: reducing flashovers
  • Spring tension: Traction / standard / North American vs. European standards
  • Brush grade and varying loads
  • Cleaning armatures
  • Compensating winding repair tips
  • Parallel paths and current: theory vs. reality